The present invention relates to registration of plural image exposures formed on a photoreceptor belt in a single pass by a plurality of LED printbars and, more particularly, to improvements in registration by use of high precision, belt hole sensors.
Image printbars used in xerographic recording systems are well known in the art. The printbar generally consists of a linear array of a plurality of discrete light emitting sources. Light emitting diode (LED) arrays are preferred for many recording applications. In order to achieve high resolution, a large number of light emitting diodes, or pixels, are arranged in a linear array on a common substrate. Each LED in the linear array is used to expose a corresponding area on a moving photoreceptor to an exposure value defined by the video data information applied to the drive circuits of the printbars. The photoreceptor is advanced in the process direction to provide a desired image by the formation of sequential scan lines.
In one embodiment of a color xerographic printer, a plurality of LED printbars are positioned adjacent to a photoreceptor belt surface and selectively energized to create successive image exposures. If two bars are used, the system typically provides one highlight color and one black color. For full color, four bars are used, one for each of the three basic colors and a fourth printbar for black images.
FIG. 1 shows a side view of a prior art, single pass, color printing system having four exposure stations 10, 12, 14, 16, each station including an LED printbar 10A, 12A, 14A, 16A. FIG. 2 shows a top view of the system of FIG. 1 absent some of the xerographic stations, for ease of description. Each printbar is selectively addressed by video image signals processed through controller circuit 15, to produce a modulated output which is coupled through a gradient index lens array 10B, 12B, 14B, 16B, onto the surface of previously charged photoreceptor belt 17. The length of belt 17 is designed to accept an integral number of full page image areas; e.g. I.sub.1 -I.sub.4, represented by dashed lines. Upstream of each exposure station are charge devices 18, 19, 20, 21, which place a predetermined electrical charge on the surface of belt 17. As the belt moves in the indicated direction, each image area moves past each of the printbars, with each bar providing its own exposure pattern, in response to the video data input. The exposure pattern begins when the leading edge of an image area reaches a transverse start-of-exposure line, represented in image frame I.sub.1 by a line 23. The exposure pattern is formed of a plurality of closely spaced transverse scan lines. Downstream from each exposure station, a development system 26, 27, 28, 29, develops a latent image of the last exposure without disturbing previously developed images. A fully developed color image is then transferred at transfer station 33, by means not shown, to an output sheet. Further details of xerographic stations in a multiple exposure single pass system are disclosed in U.S. Pat. No. 4,660,059 and 4,833,503, whose contents are hereby incorporated by reference.
With such a system as that disclosed in FIGS. 1 and 2, each color image I.sub.1 -I.sub.4 must be precisely aligned (registered) so that all corresponding pixels in the image areas are registered. Current requirements call for registration tolerances of approximately 20 microns. The printbar alignment requirements are for the pixels of each bar to be aligned in the transverse or Y-direction of FIG. 2, as well as the process or X-direction. This alignment must be maintained through continuous revolutions (passes) of the photoreceptor.
Continuing with the description of the system shown in FIGS. 1 and 2, a pair of inboard-outboard registration holes 30, 32 are provided at the beginning of one of the image frames. Additional holes 34, 36, 38 are located in an aligned row along the outboard end of the belt just upstream of the lead edge of an associated image frame. Fixed in place beneath each of the printbars are a pair of registration sensors, one at the inboard and one at the outboard end. Thus, printbar 10A has associated sensor pairs 40, 42, printbar 12A has sensor pairs 44, 46, printbar 14A, sensors 48, 50 and printbar 16A, sensors 52, 54. The sensors are used to detect the passage of holes 30, 32 coincident with the pulsing of predetermined pixels at the end of each image bar. Signals generated by each sensor pair are used to provide correction to the printbar to correct for skew registration errors and to align the bar in the transverse or scan direction. Sensors 42, 46, 50, 54 are also used to sense the passing of holes 32, 34, 36, 38 and enable precise energization of the associated printbar to form the lead edge scan line of each associated frame. Co-pending application U.S. Ser. No. 07/807,931, assigned to the same assignee as the present invention, discloses LED printbar registration techniques which utilize hole sensors to generate registration correction signals, which are used to drive stepper motors, which provide incremental rotational and transverse motion to an associated image bar. The contents of this application are hereby incorporated by reference.
In the prior art system, as disclosed in this co-pending '931 application, the sensors are typically simple interruptive optical switches consisting of an LED arranged so as to illuminate a phototransistor in a "U" shaped channel. These detectors, which also find use for locating belt holes associated with seams in a photoreceptor belt, have triggering requirements which are not very stringent. For example, a sensor XPN 130S892 will be triggered within .+-.500 .mu.m (microns) of an optical center line, when an opaque mask (a belt portion with a hole therethrough) is slowly moved into the view of the sensor. Additionally, the sensor rise and fall times are specified to be less than approximately 600 .mu.sec. (microseconds) and assuming, for example, that photoreceptor 17 is moving at a conventional velocity of 20 inches/sec., this translates into a triggering uncertainty of .+-.150 .mu.m. This uncertainty is substantially greater than the .+-.20 .mu.m registration requirement needed for color registration purposes in an LED color printer.
It is therefore an object of the present invention to improve the hole sensing mechanism used in prior art LED printers so that the accuracy of the detection is increased to that required to accurately register the lead edge of four color images. This object is realized by employing the LED printbar flux as the detector illumination means and by employing a PIN photodiode with a detection circuit which conditions the detector output signal to more precisely identify the edge transition position of the belt hole, as it passes into the view of the associated printbar sensor, thereby enabling more precise and repetitive location of each hole edge relative to the printbar exposure zone on the photoreceptor. More particularly, the present invention relates to an apparatus for forming multiple image exposure frames on a photoconductive member, moving in a process direction, during a single pass including:
a photoreceptor belt adapted to accommodate the formation of an integral number of image exposure frames, said belt having a first and second alignment aperture on opposite sides of the belt width and outside of the exposure frame, said aperture having a leading and trailing edge defined by the process direction,
a plurality of linear image printbars, each printbar associated with the formation of one of said image exposure frames, each printbar having a central portion of light emitting pixels which are selectively activated to form said image exposure frames and at least one light emitting end pixel at each end and outside of said exposure frames which is selectively activated for printbar alignment and registration purposes, each printbar having a first and second detecting means said detecting means for detecting the light from said end pixels through said alignment apertures as the belt moves there past, said detecting means generating an output signal precisely defining the trailing edge of the alignment apertures, said detecting means comprising:
a photodetector which provides an output signal representing the activated end pixels as viewed through the boundaries of the leading and trailing edge of the alignment apertures, and
a precision registration circuit which operates on said photodetector output signal to generate an output signal coincident in time with the trailing edge of said alignment apertures.